Method of chromium plating



mersed in a hot solution of an alkaline compound,"

- sodium hydroxide or the like, contained in an' Patented May 31-, 1938 PATENT OFFICE 2,113,950 METHOD or cr'mormm rmrmc Arthur Wagner, Riverside, IlLpassig'nor to, Western Electric Company, Incorporated, New York, N. Y., a corporation of New York No Drawing. Application January Serial N0. 57,922

4 Claims. (CL 204-1) This invention relates to a method of chromium plating, and more particularly to a method of chromium plating over a previously chromium plated surface or a surface of a chromium alloy.

Due to chemical changes in the surface of a chromium plated'object when exposed to the atmosphere or when the chromium plating process is interrupted, it is impossible, at leastwith certain well known electroplating processes, to consistently plate chromiumover a previously chromium plated surface or over the surface of a high chromium content alloy. v

In order to chromium plate-a previously chromium plated surface, it has been customary in many instances heretofore to completely strip an article of a previously applied chromium plate and replate the entire coating.

An object of the present invention is to provide an effective and efficient method of electroplating on a chromium surface.

In accordance with the preferred embodiment of the invention, a chromium plated surface which is to receive a further electroplated coating of chromium is placed in an electrolytic cell containing a hot solution of sodium carbonate andmade the cathode therein for one or more minutes after which the article is withdrawn and etched in a hydrochloric acid solution until the surface becomes discolored, the extent of discoloration necessary for any particular type of work can be readily determined by trial. The surface is then washed in cold water whereupon a further chromium plated coating may be applied thereto.

The invention is particularly applicable when chromium plating with the well known Sargent solution using an aqueous chromicacid and sulfuric acid solution having a proportion of from one to five grams of sulphate radical to 250 grams of chromic acid per liter of solution. The chromium surface to be chromium plated ls first im such 'as sodium carbonate, trisodium phosphate,

electrolytic cell. The article to beplated is-made the cathode ofthe cell and a potential preferably of about 6-18 volts is impressed upon the elec trodes while the article is in the alkaline solution.

as a result of which hydrogen is liberated at the surface of the article and acts as a reducing agent for anyoxides that are present. There also appears to be a beneficial hydrogen bombardment of the surface which exposes the chromium by removing a passive film therefrom. This process is continued for one or more minutes after which the article is withdrawn from the bath and is etched in a suitable acid solution, such as a solu tion of hydrochloric or sulphuric acid. This treatment results in the formation of a black smut comprising chromium chloride or chromium sulphate on the surface of the article and after removal from theacid solution the object is washed well in cold water and placed in the chromium plating bath. The smut is not removed during washing but acts as a protective layer on the surface until the article is placed in the plating bath where'it is removed by-the plating process before plating begins. The plating process is then car ried out in the usual manner. I

The process lends itself admirably to the repair of defective chromium plated work, such as parts which have not been entirely covered in a previous plating operation or parts which have become defective or worn in service. Also chromium plating operations on parts requiring close limits for the thickness of deposit can be controlled very accurately since the part can be removed from the plating solution, the thickness of the deposit determined and an additional deposit applied if necessary. In some cases it is also found that when work cannot be completed during the r'egular working day, the parts may be treated in accordance with the invention the following day and the plating continued.

The invention also makes it possible to electr of substantially pure chromium, but also a surface of high chromium content such as is presented by an alloy containing chromium. a

What is claimed is: v

'1. A method of chromium platingon a chromium surface which comprises placing the article to be plated in a bath consisting essentially of a solution of an alkaline compound, passing- .a current through the solution with the article as a cathode, immersing the article in an acid solution of the group consistingof hydrochloric and sul-g phuric acids until a black smut is formed on the surface of the article, and electroplating the article in a chromic acidbath.

- 2. A method of chromium plating on a chromium surface which comprises placing the article in a bath consisting essentially *of an alkaline solution of a sodium compound passing a current through the solution with the article as a cathode,

immersing the article in an acid solution of thegroup consisting of hydrochloric and sulphuric acids until a black smut is formed on the surface of the article, and electroplating the article in a chromic acid bath.

3. A method of chromium plating on a chromium surface which comprises cathodically treating the article in a bath consisting essentially of a solution of an alkaline compound, immersing 

